The electrochemical promotion of the decomposition of NO in the presence of excess O-2 was carried out by applying AC voltages lower than fi V at frequencies from 0.01 to 10(3) Hz to a YSZ cell having two same-metal electrodes, Au, Pd, Pt or Rh. The Pd and Pt electrodes were relatively effective for this reaction. This could be explained by their electrochemical oxygen-pumping properties and catalytic activities for the reduction of NO. The promotional effect was the most enhanced at a frequency of 0.1 Hz and became larger as the concentration of NO increased or the concentration of O-2 decreased. Most of the product was N-2, although a small amount of N2O was observed at low conversion of NO. The comparison of AC electrolysis with DC electrolysis showed that the former had a somewhat lower current efficiency for the decomposition of NO than die latter at lower than 3 V, above which the opposite result was observed. In addition, DC electrolysis brought about a significant deterioration of the cell, but AC electrolysis did not result in such a deterioration even at a high voltage of 4 V.